The geometry optimization for the pyridine N-oxide/trichloroacetic acid complex is realized in B3LYP/cc-pVTZ approximation. Computations of IR absorption spectra for the optimized configuration are performed in harmonic and anharmonic approximations. It is found that taking into account the anharmonicity effects leads to the red shift of the stretching vibration frequency of O-H bond by more than 600 cm-1. At the same time, the red shift of the in-plane and out-of-plane bending vibrations of the hydroxyl group does not exceed 70 cm-1 . The 1D potential curves and 2D potential surfaces associated with stretching and bending vibrations of a hydroxyl group are computed. The vibrational frequencies of O-H bonds are derived by numerical solution of Schrödinger equations. The frequencies are compared to those of anharmonic computations for this complex. A combined method including the advantages of both approaches is proposed.